1. Field of the Invention
The present invention relates generally to an air conditioning system and, in particular but not exclusively, to a multi-room air conditioning system having one outdoor unit and a plurality of indoor units connected thereto wherein refrigerant circulation is controlled by a corresponding number of motor-driven expansion valves.
2. Description of the Related Art
Recently, a multi-room air conditioning system having a plurality of indoor units connected to a single outdoor unit is being gradually accepted to consumers of ordinary homes in view of a reduced outdoor space and the appearance. Compared with the case wherein a plurality of single-room air conditioners each having only one indoor unit connected to one outdoor unit are installed, the multi-room air conditioning system is advantageous in respect of the cost, resulting in a gradual increase in demand of consumers.
In the multi-room air conditioning system, the capacity of a compressor is controlled according to the total demand of the indoor units, while the opening of a flow control valve connected to each indoor unit is independently controlled according to the demand of the corresponding indoor unit.
However, because opening control of the flow control valves is separated for each indoor unit, various problems arise when there is a great difference in the required demand of the respective indoor units during heating mode operation.
By way of example, flow control by the flow control valves is carried out on downstream sides of the indoor units and, hence, a large amount of liquid refrigerant is apt to be collected in an indoor unit having a small demand load, giving rise to shortage of the refrigerant circulation in the whole refrigeration cycle. This in turn causes a problem in that control of only the opening of each motor-driven expansion valve cannot maintain the degree of superheat of the refrigerant substantially constant at a refrigerant heater outlet.
To solve this problem, if the heating capacity is reduced by reducing the output of the refrigerant heater or the ratio of heating capacity between the indoor units is reduced by increasing the minimum opening of the flow control valves, there arises a problem in that the specification of the air conditioning system is lowered.
In view of this problem, Japanese Laid-Open Patent Publication (unexamined) No. 5-26530 discloses an air conditioner capable of always ensuring an appropriate heating capacity by maintaining the degree of superheat of a refrigerant to a predetermined value at an outdoor heat exchanger on the heating source side even if there is a great difference between the heating demands of indoor units.
This air conditioner is provided with a refrigerant heater having a gas burner, a combustion fan, a proportional valve, an igniter, a flame detector and the like, wherein the refrigerant is heated by flames of the gas burner during heating mode operation. During the heating mode operation, if the difference between the demand loads of the indoor units is greater than a predetermined value, a two-way valve corresponding to an indoor unit having a greater demand load is opened, while a two-way valve corresponding to an indoor unit having a smaller demand load is closed. Concurrently therewith, the opening of flow control valves is controlled so that refrigerants entering respective indoor heat exchangers may be in a predetermined temperature relationship depending on the demand loads of the indoor units. Also, the opening of each motor-driven expansion valve is controlled while the total opening of all the motor-driven expansion valves is maintained constant so that the degree of supercooling may become equal to each other at the indoor heat exchangers.
Although a multi-room air conditioning system of a type wherein refrigerant is heated by an economical oil is now being examined, control of an oil heater is not easy compared with refrigerant heating with the use of flames of a gas burner because a large amount of refrigerant is filled in the system having a plurality of indoor units connected to a single outdoor unit. It is particularly important to control a balance between refrigerant heating and refrigerant circulation. In the multi-room air conditioning system, there is a significant change in refrigerant circulation. When the heat added exceeds the heat dissipation capacity of refrigerant circulation, an abnormal rise in the outlet temperature of the refrigerant heater or a rise in exhaust gas temperature occurs. In contrast, when refrigerant circulation exceeds the heating capacity of the heater, a drop in compressor reliability or a rise in the input to the compressor occurs.
Furthermore, a material having a relatively large heat capacity such as, for example, aluminum is employed in a combustor of the oil heater and, hence, the refrigerant temperature changes only a little even if the heat added changes considerably, making it difficult to control refrigerant heating.